The standard seems to be whether the statement clothing makes will expression will "create a substantial disruption or material interference with school activities or will invade the rights of others."
So the federal courts have allowed a student in West Virginia to wear Confederate flag clothing to school and forbidden it in an Indiana district. In the first case, there wasn't much tension in the school and African-American students weren't offended. In the second case, the rule barring such clothing was adopted because of conflicts and disruptions at the school.
The big case was Tinker v. Des Moines Independent Community School Dist. (1969) and involved students wearing black armbands to protest the Vietnam war. It looks like schools won't be able to ban political speech at least until the country is as divided as it was over Vietnam, but if the school does have a hard and fast rule against wearing clothes with obscenities written on them it could probably disallow a shirt with obscenities about the President printed on it.
You've done your homework and thanks for the case cite.
In my common sense world, even in a controlled environment (read public school) dissent or controversy ought to be allowed unconditionally as the 1st is applied (I am aware of the common restrictions on Amend #1 as to pornography etc...).
So specific to this issue I don't agree with school districts suppressing any activity that is not to the direct detriment of student safety.
If one is offended, then turn away as I don't believe the perceived rights of the offendee overrule the rights of the offender.
But I'm not a judge so my opinion holds weight with my family and my pets:)